There is a need in the field(s) of optical engineering for red, green, and blue lighting systems in various devices, applications, and technologies. Among these technologies are backlighting sources for display systems, such as plasma displays, and warm-white light sources in general lighting.
Various configurations of LED light sources and light-emitting phosphors are possible in the design of such RGB systems. The conventional one, shown schematically in FIG. 1A, employs three light emitting diodes (LEDs). Each of the three LEDs is a semiconductor-based integrated circuit, or “chip”, and there is one chip for each of the red, green, and blue colors. A disadvantage of the conventional system is that a separate electrical current controller is needed for each LED, like in many situations, it is better to have as few current controllers in the system as is possible. The prior art system of FIG. 1A requires three circuit controllers.
The RGB systems depicted in FIGS. 1B-D differ from that of FIG. 1A in that they have at least one photoluminescent substance (a “phosphor”) substituting for at least one of the red, green, or blue LEDs of the system. The progression from FIGS. 1A-1D shows the effect of replacing LEDs with phosphors; in no particular order regarding color, first the green LED is replaced with a green phosphor (going from FIG. 1A to 1B); then the red LED is replaced with a red phosphor along with the green (going from FIG. 1B to 1C); finally, all three visible light emitting LEDs are replaced in FIG. 1D with phosphors, although a UV-emitting LED has been added to this system to provide excitation radiation to the phosphors. Thus, FIG. 1D depicts a strategy that is in a sense somewhat opposite to that of FIG. 1A, in that each of the RGB colors in FIG. 1D is provided by a phosphor, and therefore the excitation source is a non-visible, UV emitting LED. The practice of technologies based on a UV excitation source are somewhat further away from commercialization than those based on blue LEDs, but nonetheless, the configuration of FIG. 1D is still one in which the present nitride-based red phosphors may be used.
This means that no blue phosphors are used in the embodiments of FIGS. 1B and 1C because the blue LED provides the blue component of the light needed in various applications like backlighting and warm-white light general lighting. In this regard the blue LED is unique in the system because it serves a dual role; in addition to providing blue light to the final light product, it provides excitation to either or both of the red or green phosphors in the system. Systems such as those depicted in FIGS. 1C and 1D are the subject of the present disclosure; these configurations are particularly suited for silicon nitride based red-emitting phosphors.
Since earlier versions of these red phosphors were based on nitrides of silicon, they may be generically referred to as “nitride-based” silicates, or nitridosilicates. Newer versions have included aluminum such that the resulting compounds are referred to as “nitridoaluminosilicate nitrides.” The deliberate inclusion of oxygen into these crystals in a desired stoichiometric manner gives rise to a certain class of red-emitting phosphors, and compounds known as “SiAlONs” can also be in some cases sources of green and yellow-green illumination. When oxygen substitutes for nitrogen the resulting compound may be described as an “oxynitrides.”
As alluded to earlier, a combination of LED-generated blue light, and phosphor-generated green and red light, may be used to generate the white light from a so-called “white LED.” Previously known white light generating systems used a blue LED in conjunction with a yellow emitting, cerium-doped, yttrium aluminum garnet known as “YAG,” having the formula Y3Al5O12:Ce3+. Such systems have correlated temperatures (CCTs) of greater than about 4,500 K, and color rendering indexes (CRIs) ranging from about 75 to 82. The blue emitting LED provides excitation radiation ranging from about 400 to 480 nm.
One way of achieving a flexibility of design in blue LED-based devices involves creating a wider separation between the yellow and/or green phosphors, and the red phosphors, the phosphors relative to one another in CIE space. CIE coordinates will be discussed further later in this disclosure, but suffice it to say for now that “CIE space” means the area in a triangle mapped by two vertices of a triangle defined by phosphors, and the third by the blue LED. A yellow and/or green apex widely separated from that of the blue LED create a rich diversity of components for white light generation.
As described in U.S. Pat. No. 7,252,787 to D. Hancu et al., red sources were used with YAG and TAG-based yellow sources to produce a high color rendering index have included nitrides having the formula (Ba,Sr,Ca)xSiyNz:Eu2+, where each of the x, y, and z parameters was greater than zero. A disadvantage of such phosphors used with YAG/TAG was that they reabsorb emissions from those phosphors due to overlapping of the Eu2+ absorption bands with the emission of the (Tb,Y)3Al5O12:Ce3+) phosphors. Thus, there is a need for red phosphors having a redder emission than these nitrides to produce white light illumination with high CRI.
Host lattices for new red-emitting phosphors based on nitridosilicate compounds were introduced in the mid-1990's. Such phosphors have desirable mechanical and thermal properties due to a three dimensional network of cross-linked SiN4 tetrahedra in which alkali earth ions (M=Ca, Sr, and Ba) are incorporated. The formula used in U.S. Pat. No. 6,649,946 to Bogner et al. to describe such phosphors was MxSiyNz, where M was at least one of an alkaline earth metal, and where z=2/x+4/3y. The nitrogen of these nitrides increased the content of colvalent bonding, and thus ligand-field splitting. This lead to a pronounced shift of excitation and emission bands to longer wavelengths in comparison to oxide lattices.
The effect of the alkaline earth component of such nitridosilicates when y is 5 was investigated by Y. Q. Li et al. in “Luminescence properties of red-emitting M2Si5N8:Eu2+ (M=Ca, Sr, Ba) LED conversion phosphors,” J. of Alloys and Compounds 417 (2006), pp. 273-279. Polycrystalline powders were prepared by a solid state reaction mechanism. The crystal structure of the Ca-containing member of this family was monoclinic with space group Cc, whereas the Sr and Ba members were isostructral with orthorhombic space group Pmn21. There was a formation of a complete solid solution between the Sr and Ba end-members in the latter compound(s).
As taught by Li et al., the excitation spectra are not substantially dependent on the type of alkaline earth, but the position of the emission bands are. The peak emission bands for a 1 mole percent activator concentration were 605, 610, and 574 nm, for M=Ca, Sr, and Ba. The shift in the emission band with the nature of the alkaline earth is due to a difference in the Stokes shift for each of the members, where the Stokes shift gradually increases with the sequence Ca>Sr>Ba, and this trend is predictable if one observes that the relaxation of the 4f65d1 state becomes less restricted when the size of the alkaline-earth ion decreases. Further, the Stokes shift increases for as the Eu concentration is increased in all cases.
US 2007/0040152 elucidated the difficulties in producing a nitridosilicate based compound such as M2Si5N8, MSi7N10, and MSiN2, where M is at least one element selected from Mg, Ca, Sr, and Ba, etc., and where the compound contains substantially no oxygen. This may be achieved, it is taught, by using as starting materials the nitrides of the alkaline-earth elements and the rare earth elements, but these nitrides are difficult to obtain, expensive, and difficult to handle. These factors conspire to make nitridosilicate-based phosphors difficult to produce industrially. As stated by the reference: “the conventional nitridosilicate-based compound has the following problems: (1) low purity due to the presence of a large amount of impurity oxygen, (2) low material performance of a phosphor caused by the low purity; (3) high cost; and the like.” The problems included low luminous flux and brightness.
What is needed in the art are red-emitting phosphors in red, green, and blue (RGB) lighting systems for use in backlighting displays and warm white-light applications, where the red phosphors have high luminous flux and brightness. The present disclosure describes improvements in red-emitting phosphor based on CaAlSiN3 type compounds activated with divalent europium. In conjunction with phosphors emitting at other wavelengths, it is believed the present embodiments provide general illumination sources having higher CRIs and lower CCTs than those currently available.